Yom Kippur is probably the most important holiday of the Jewish year. Many Jews who do not observe any other Jewish custom will refrain from work, fast and/or attend synagogue services on this day. Yom Kippur occurs on the 10th day of Tishri. The holiday is instituted at Leviticus 23:26.
The name "Yom Kippur" means "Day of Atonement". Yom Kippur during biblical times was not an ordinary day. The sacrificing on this day was not like any of the other sacrifices performed during the year. This once a year sacrifice required that the Kohen HaGadol (High Priest) enter the Kodesh Hakodashim (Holy of Holies) alone, it was just him and God. He took with him a vessel containing the blood of the goat upon which the sacrificial lot fell. This blood was then sprinkled on the kaporet (Mercy Seat), which covered the Aron HaKodesh (Ark of the Covenant) found in the Kodesh Hakodashim. If acceptable, that is if the animal sacrificed was without spot or blemish, God received the offering and declared the sins of b'nai Yisreal (the children of Israel) atoned for.